Part 6 · Capstone: Design Your Own Subsistence Farm
Water Design
Budget your site's rainfall and lay the swales, ponds and tanks that store it high and spend it by gravity.
On a slope, water is not an input you buy. It is the whole economy, and the design of the farm is mostly the design of the water. The third deliverable is a water budget and a water plan: the arithmetic of what falls and what grows, and the drawing of where the water is caught, held, and spent.
What you produce
Two things. A water budget table, and a water plan drawn on the contour map from Lesson 602. The budget is the arithmetic and it must close. The plan is the layout: every swale, pond, tank, and overflow route, each one on a marked contour, each one with a job.
The exact format
The budget table, one row per land use:
| Land use | Area (m²) | Rainfed or irrigated | Demand (m³/yr) |
|---|---|---|---|
| Staples | rainfed | 0 | |
| Fruit and nut | rainfed | 0 | |
| Vegetable garden | irrigated, dry months | ||
| Site | rain harvest | rainfall × area |
The supply line is rainfall times area. If your site is 929 square metres and the rain is 700 millimetres, then 929 × 0.7 = 650 cubic metres fall on the ground in a year. Almost all of that you will not store; you will spread it and sink it where it falls. The plan shows the difference between the rain that falls, the rain you catch, and the rain you must pump.
The plan itself carries five elements: the swale or fanya juu ditch lines on the contour; the high pond or tank that gathers them; the distribution run down through the terraces by gravity; the overflow that leads surplus to the next store rather than off the site; and the shade planted over every open store, because a store you let the sun bake away is a store you never had.
The rule that makes it work
The design goal is the one the house design states plainly: store it early, store it high, spend it slowly, spend it by gravity. The staples are rainfed. The only irrigated ground is the small vegetable garden, and only in the dry months. Everything else the sky carries.
Worked example at the 10,000 square foot scale
Our 10,000 square feet is 929 square metres. At 700 millimetres of rain, 650 cubic metres fall on the site each year. That is the whole supply, and it arrives for free.
The demand is small if the design is right. The reference farm irrigated a 140-square-metre garden with 50 cubic metres of water a year. Scale that garden to our 15 percent fruit and nut plus 5 percent vegetable, and give the vegetables 500 square feet, which is 46 square metres. The garden then needs 50 × 46 ÷ 140, about 17 cubic metres, or just under three percent of the rain that falls on the whole site. The 8,000 square feet of staples and the 1,500 of nut and fruit take no irrigation at all. They feed from the rain that sinks in behind the fanya juu bank.
The plan puts the swales on the contours from Lesson 602, the pond on the high corner where the top-left water enters, the distribution run down the south face to the garden at the bottom, and the overflow leading the surplus into a second swale below rather than into the hedge gap. In the worst dry spell a single small solar pump lifts water back to the high tank, and gravity carries it down again. Pelman proved that a person can live on rain alone; the pump is only for the fortnight that tests the plan.
The test
The water plan passes if three things are true. The staples are rainfed. The irrigated garden is a small fraction of the whole, not the whole. And no water leaves the site until it has been stored at least twice. If your plan shows a pipe from the mains carrying the staple crop, it has failed, and no amount of swale will hide it.
Exercise
Compute your own water budget
Measure your site's area in square metres. Multiply by your annual rainfall in metres. That number is the rain that falls on you in a year. Then estimate the demand of your garden alone, using the reference figure of about 0.36 cubic metres of irrigation water per square metre of garden per year. If your garden demand is more than a tenth of the rain that falls, your garden is too big or your climate is wrong for this design.
Fieldwork
Dig the drainage test holes
Dig three test holes a spade deep, one at the top of the slope, one midway, one at the bottom, and fill each with water. Time how long the water takes to drain away and write the three times down. Watch where the run-on gathers on the surface while it rains and mark each spot with a stake. Bring back the three drain times and the staked line where the water runs.
Failure mode
The failure mode is a water budget that does not close and a plan that hides the gap. If the staples need irrigation, the design has already failed, and a swale drawn on paper will not hold a summer. Count the rain that falls, catch and store it high, and spend it by gravity, or the farm is a mains connection with a story.
The figures for the reference farm, 740 square metres, 700 millimetres of rain, and 50 cubic metres of irrigation for a 140-square-metre garden, come from the published study of Alik Pelman's farm in western Galilee. They are one case, not a law. Read your own rain gauge for a year before you trust any of them on your ground.
